In the field of fabrication of large-sized nuclear components to high quality, mechanical precision and reliability requirements, the use of welding technologies aimed at reducing to a minimum or even completely eliminating the filler metal are arousing increasing interest, due to the undisputable technical and economic advantages deriving from the lower residual stresses after welding and the consequent greater dimensional stability.
Among the many components of a nuclear plant, the case of the diagrid supporting the core of fast-neutron plants is particularly significant.
This component has the fundamental function of conveying the Sodium at a pressure inside the fuel assemblies forming the CORE, and thus of keeping these assemblies in a particularly precise vertical position.
The component is immersed in Sodium at a working temperature which generally varies between 400.degree. and 500.degree. C. Up until the present time, the tendency of manufacturers of this type of component has been towards structures designed basically to be mechanically assembled and bolted together.
Since access to components of this type is impossible during the 30 year life-span of the power plant, and due to the very stringent requirements as far as concerns mechanical strength, precision and geometrical stability, as well as their particularly complex geometry, the considerable thickness of the ring (80 mm) needed to fasten the bolts and the high density and weight of the bolts, a remarkable work burden is entailed in terms of both calculations and fabrication. This, of course, leads to very high overall costs.
This technical and economical aspect may be justified for a prototype plant, but not for the development of a series of reactors.